The present invention relates to a brake system for use in an automatic transmission and, more particularly, to a band brake.
An automatic transmission for automobiles employs two or three sets of planetary gear mechanisms or differential gear mechanisms, as is well known in the prior art, for establishing a suitable gear ratio by fixing one of the three elements of each gear mechanism (e.g., a sun gear, a carrier and a ring gear in the case of the planetary gear mechanism, or a ring gear and a pair of meshing side gears in the case of the differential gear mechanism) while using the other two elements as input and output members. One example of this structure is schematically shown in FIG. 16, in which an automatic transmission is intended to establish speed changes of three-forward and one-reverse gear ratios by using two sets of planetary gear mechanisms 1 and 2.
An input shaft 3 to be connected to an engine (although not shown) is united with the pump impeller 5 of a torque converter 4. The pump impeller 5 is faced across a stator 6 by a turbine runner 7, which is connected to the input shaft 8 of the automatic transmission. This input shaft 8 is connected to the ring gear 9 of the rear planetary gear mechanism 2 through a first clutch C1 and an intermediate shaft 10 and to a sun gear shaft 11 through a second clutch C2. The sun gear shaft 11 carries the sun gears 12 and 13 of the planetary gear mechanisms 1 and 2, respectively. Between the sun gear shaft 11 and a stationary portion such as the transmission casing, there are serially arranged a first one-way clutch Oc1 and a first brake B1 so as to block the revolutions of the sun gear shaft 11 in one predetermined direction. In order to block the revolutions of the sun gear shaft 11 selectively, moreover, there is arranged a second brake B2 acting as a band brake, which is disposed around the outer circumference of such a clutch drum of the second clutch C2 as is united with the sun gear shaft 11. A ring gear 14 of the front planetary gear mechanism 1 and a carrier 15 of the rear planetary gear mechanism 2 are connected to each other and to an output shaft 16. The front planetary gear mechanism 1 has its carrier 17 connected to both a second one-way clutch Oc2 for blocking its revolutions in one predetermined direction and a third brake B3 acting as a band brake for selectively blocking its forward and backward revolutions.
The speed changes to be set by the automatic transmission described above are enumerated in Table 1, in which: circles indicate the state of "in engagement"; blanks indicate the state of "out of engagement"; and parenthesized circles indicate the state of "in engagement" while the automobile is being braked by the engine.
TABLE 1 ______________________________________ Clutches Brakes One-Way Clutches C1 C2 B1 B2 B3 Oc1 Oc2 ______________________________________ 1st Gear O (O) O 2nd Gear O O (O) O 3rd Gear O O Reverse O O ______________________________________
In the forward first gear ratio, as seen from Table 1, the first clutch C1 is engaged to connect the ring gear 9 of the rear planetary gear mechanism 2 to the input shaft 8. In this case, a torque in the reverse direction (which is opposite to the revolving direction of the input shaft 8) is applied to the carrier 17 of the front planetary gear mechanism 1 but is blocked by the second one-way clutch Oc2 to cause the carrier 17 to act as the stationary element so that the gear ratio takes a high value. If, however, the braking force is to be effected in that state by the engine by throttling the accelerator opening while the vehicle is running downhill, for example, the carrier 17 is allowed to freely revolve by a forward torque coming from the output shaft 16 so that the ring gear 9 of the rear planetary gear mechanism 2 acts as the stationary element to render the braking force by the engine ineffective. Therefore, the braking effect by the engine is recovered by engaging the third brake B3 to use the carrier 17 as the stationary element and by using the ring gear 9 of the rear planetary gear mechanism 2 as the output element.
In the case of the forward second gear ratio, on the other hand, the first clutch C1 is engaged to connect the ring gear 9 of the rear planetary gear mechanism 2 to the input shaft 8, and the first brake B1 is engaged to block the reverse revolutions of the sun gear shaft 11. As a result, the carrier 15 of the rear planetary gear mechanism 2 is revolved in the forward direction together with the revolutions of the pinion gear meshing therewith so that the gear ratio takes a value lower than the forward first one. If, in this case, the input comes from the output shaft 16, the forward torque is applied to the sun gear shaft 11 so that the sun gear shaft 11 is revolved in the forward direction at an accelerated speed while causing the ring gear 9 of the rear planetary gear mechanism 2 to act as the stationary element. While the automobile is being braked by the engine, therefore, the second brake B2 is engaged to block the forward and backward revolutions of the sun gear shaft 11.
In the case of the forward third gear ratio, still moreover, the first and second clutches C1 and C2 are engaged. In this case, the rear planetary gear mechanism 2 is united as a whole so that the gear ratio takes the value "1".
In the reverse case, on the other hand, the second clutch C2 is engaged to connect the sun gear shaft 11 to the input shaft 9, and the third brake B3 is engaged to fix the carrier 17 of the front planetary gear mechanism 1. As a result, the revolutions of the sun gear 12 of the front planetary gear mechanism 1 are reversed and transmitted to the ring gear 14 to establish a reverse state decelerated in accordance with the ratio of those gears.
Incidentally in the aforementioned automatic transmission, as is apparently seen from Table 1, the second brake B2 is engaged only when the automobile is braked by the engine. This engagement comes from the provision of the first one-way clutch Oc1. It is, therefore, conceivable that the first one-way clutch Oc1 is substituted by the second brake B2 to omit the first one-way clutch Oc1 and the first brake B1. This concept is exemplified in FIG. 17, and its operations are tabulated in Table 2 but are identical to those of the automatic transmission shown in FIG. 16 except the fact that the second brake B2 never fails to be engaged in the case of the forward second gear ratio.
TABLE 2 ______________________________________ Clutches Brakes One-Way Clutches C1 C2 B2 B3 Oc2 ______________________________________ 1st Gear O (O) O 2nd Gear O O 3rd Gear O O Reverse O O ______________________________________
Generally speaking, the automatic transmission has a major technical target to change its speeds as smoothly as possible. For this target, it is sufficient to increase the number of speed changes and accordingly the number of planetary gear mechanisms required. This requirement raises another problem that the automatic transmission will increase its weight and size. As shown in FIGS. 16 and 17, therefore, the one-way clutches Oc1 and Oc2 are used to effect automatic disengagement for the speed changes thereby to prevent the resultant shocks. Since, however, the one-way clutches effect the engagements only in one direction, as named so, they cannot function in case the torque is reversed as in case the automobile is to be braked by the engine. Hence, the structure shown in FIG. 16 requires the second brake B2, which raises one cause for increasing the weight and price.
On the contrary, the structure shown in FIG. 17 can drop the weight and cost because it omits the first one-way clutch Oc1 and the first brake B1 from the structure shown in FIG. 16. Since, however, the second brake B2 is released for the speed change from the forward second to third speed, as tabulated in Table 2, the releasing timing is difficult to set, thus causing another problem that the structure and adjustment of the hydraulic circuit inclusive are complicated.
In the automatic transmission using a band brake such as the aforementioned second brake B2 and third brake B3, there has been proposed in Japanese Utility Model Laid-Open No. 61-124742 a brake having a built-in shock absorber for lightening the shocks which are caused by engaging that band brake. In the structure of this proposal, a support stem for holding one end of the brake band and a cylinder tube of a hydraulic servo-cylinder for generating a fastening force are arranged to move back and forth in the tangential direction of the revolving member and are connected and united together, and an elastic member is interposed between the support stem and the stationary portion such as a casing. With this structure, therefore, the support stem and the cylinder tube are moved together by the revolving force of the revolving member, when the braking force is applied by actuating the hydraulic cylinder to fasten the brake band, but the elastic member can exert a shock absorbing action upon the movements to avoid an abrupt braking motion and absorb the speed changing shocks. However, this shock absorbing action is caused only when the brake is to be engaged but cannot absorb the shocks of the speed change accompanied by the release of the brake.